Line a baking sheet with parchment paper. Set aside.
In a heavy saucepan over medium heat, combine the sugar, corn syrup, and water. Stir until the sugar dissolves.
Bring to a boil and cook, without stirring, until the mixture reaches 300°F (149°C) on a candy thermometer (hard crack stage). This will take about 10-15 minutes.
Remove from heat and stir in the butter and vanilla extract. Be careful, as the mixture will bubble.
Add the toasted pecan halves and stir to coat evenly.
Drop spoonfuls of the pecan mixture onto the prepared baking sheet.
Let cool completely before serving.
Notes
Store in an airtight container at room temperature.